Biology
Investigation:

The
Response of Blowfly Larvae to Light

Background Information

This section contains general information about blowflies relevant to this
investigation, leading to the reasoning of a prediction in the next section.

Blowflies

Blowflies are large-eyed flies, e.g.
bluebottles Calliphora spp. and
greenbottle Lucilia sericata. They
lay their eggs in decaying meat and other foodstuffs, or in the case of
greenbottle and L. cuprina (shown in Figure 5 on Page 13) in living sheep!
The lifecycle of the fly that I will be using for my experiments (bluebottles)
is shown in Figure 1. The numbers on
the inside indicate the approximate days elapsing. Bluebottle is a name given
to two similar species of true flies, Calliphora
erythrocephala and C. vomitora.
They have a four-stage life (metamorphosis) cycle, consisting of an egg stage,
hatching into a larval stage, which metamorphoses into the pupa, from which the
adult (imago) eventually hatches.

Each female bluebottle fly lays about 600 eggs,
which hatch in about a day. Under favourable conditions the larvae pupate after
in-between four days and a week, and emerge as adults a fortnight later, after
twelve to seventy-two hours as pupae. This is summarised in Figure 1.

As the larva hatches out of the egg, it’s sole
purpose is to feed so that it has enough energy for the metamorphosis during
the non-feeding pupal stage. At the larval stage, a developing bluebottle has
no protection from radiation, and it's white skin would make it stand out
strongly to predators. Later towards the pupal stage it develops protective
pigment, and changes to a less conspicuous brown colour.

As the eggs are laid in a foodstuff, it is only
natural that it would be beneficial to the larva to stay there in the
nutrient-rich, insulated, moist, and predator-free environment until it has
enough energy and it pupates, and develops into a fly, when it requires to be
in the open. It has, needless to say (nature is almost always perfect), evolved
a mechanism to satisfy these changing needs.

Responses to
light

Innate
behaviour, or instinct, is generally taken to be pattern of behaviour
elicited by specific stimuli and fulfilling vital needs of an organism. It is
demonstrated in its purest form by many lower animals, including insects.

The two types of
behaviour in response to light which insects are capable of are:

·Photokinesis-
the increase of rate of movement
and/or change in direction of
movement of the whole organism

Blowfly larvae have photoreceptors on each side
of their heads, as shown in Figure 2.
To gauge from which direction light comes, they move their heads from one side
to the other. If they were to merely increase their rate of movement and/or
change in direction of movement with light intensity (a kinesis), should they be on the surface, or reach the surface, they
would instantly make themselves vulnerable to predators. This kind of movement
would be too easy for predators to spot, although it stops when the organism is
buried deep enough, to be of a good advantage to them. A change in allele
frequencies resulting from their vulnerability in this situation would rather
favour the evolution of direct movement away from the light (taxis). The ideal would be if the
movement was away from the light source, and slowed down as the maggot reached
the centre of its abode, where the light intensity was at its least, so that it
stayed there. This is a negative phototactic response

Prediction

Based on the reasoning above, here is what I
think will happen.

I
therefore predict that the larvae will demonstrate a negative phototactic
response. This will mean that they will:

·Move away from a light source

·Move away faster, the greater the intensity of the
light

·Move away at a smaller angle to the direction of the
light, the greater the intensity of the light

·Respond more strongly to more damaging (higher
frequency and therefore energy) wavelengths, i.e. those closer to the
Ultra-Violet end of the spectrum of visible light.

Plan

To investigate the validity of the above
theory, I have devised the following plan.(The numbers in superscript reference their justification
later in this document.)

OUTLINE: I intend to take five blowfly larvae
and subject them to high and low light intensities and different frequencies,
and record the direction and magnitude of their response.

Chronological order of the experiment:

·Black out room, so that there is just enough light to
see by, but not enough to affect experiment.(8)

·Get five maggots out of the fridge, as needed for one
experiment(9)

·Allow them to warm to room temperature(10)

·Set the variable resistor to the correct intensity, or
fit filters, using light meter to check the value of the light intensity(11)

·Switch lights in room off

·Pick a larva up using the narrow paper scoop, with the
head at the front end

·Place the larva in the centre point of the base of the
box, its right side facing the light source, shutting the lid and starting the
timer at the same time as switching the lamp on

·After 10, 20, and 30 seconds(12), note the
position of the larva on the grid by looking through the peephole, recording
the co-ordinates on the table shown below.

·Do this with all 5 larvae(13)

·Repeat this with red, green and blue filters, alternately,
using the voltage pack to vary the output of the bulb, ensuring equal intensity
readings on the light meter(14)

·Repeat this with unfiltered light of 2, 3, 4 & 5
times the light intensity(15)

This is the table on which I will record my
results for each colour/intensity:

Larva

Quality

Temperature

Co-ordinates 10

Co-ordinates 20

Co-ordinates 30

1

White

2

White

3

White

4

White

5

White

1

Red

2

Red

3

Red

4

Red

5

Red

1

Blue

2

Blue

3

Blue

4

Blue

5

Blue

Larva

Intensity

Temperature

Co-ordinates 10

Co-ordinates 20

Co-ordinates 30

1

1

2

1

3

1

4

1

5

1

1

2

2

2

3

2

4

2

5

2

1

3

2

3

3

3

4

3

5

3

Etc.

I will carry out the following statistical analysis
on my results (on the spread sheet, together with the results):

·Use trigonometry to calculate the angle at which the
larvae moved initially, intermediately, and finally(16), the
direction being in the form of the number of degrees by which the path of the
larva differed from the direction of light

Tan q
= X-component of movement

Y-component of movement

·Compare my results to the outcome predicted
by the null hypothesis using the chi-squared test to check their validity.(17)

To do this I must first use the results of the above
trigonometrical calculation to split the responses up into groups: those which
showed positive phototaxis, those showing negative phototaxis, and those
showing a neutral response, as shown in Figure
3..(17) The direction marked as positive is the one facing the
light- the direction towards 40 on the 1cm2 grid. Positive and
negative are 120°, the
two neutral areas 60°.

The Chi-squared test

This can be
used to analyse results for the degree of variation between a set of data and
the expected outcome to show the validity of conclusions. I will use it to
compare the results that I get to the results that I would expect if the
movement of the larvae were purely random. The size of the number calculated
will tell me how sure I can be that it is not purely random and therefore that
it is reasonable to conclude an affect of the light on the larvae.

The equation is:

Where:x2= Chi-squared value

Σ=
“sum of“

O=
Occurrence of one species together with another

E= Expected
value for the latter

As the angle is the same for each
the positive, negative and neutral areas (see Figure 3), the null hypothesis would predict that an equal number
of maggots would go in each of the directions positive, negative, and neutral. The degree of confidence can then be read off
a graph such as that shown in Figure 4.
This shows that if the probability is bigger than 3.84, I can be 95% sure that
there is a significant difference my readings and the results predicted by the
null hypothesis, and the null hypothesis can be rejected.

·Use standard deviation to check the validity of my
results, as explained below.(18) This will also give me comparable
values of the randomness of the data with a decrease in intensity, which may be
a measure of how easily the maggots detect the light.

Standard Deviation

This is used to analyse the
validity of the results. The greater the standard deviation of a set of data,
the less valid the mean value is.

The equation for S.D. is:

Where:s= standard deviation

S=
“the sum of“

n2 = the individual values of
n, the angle or speed

N2=
the mean of the values of n (the sum of them divided by the number of them),
squared

x=
number of values of n in the data

Therefore the
standard deviation is the route of ((the sum of all values of n2
minus the mean of n2) divided by x, the number of values of n in the
data).

I will plot the following graphs of my results:

·The variation of speed of movement (the velocity is
worked out from the distance moved divided by the time taken) with light
quality (velocity-wavelength)(19)

·The variation of direction of movement with light
quality (degrees from normal-wavelength).(20)

·The effect of increasing the light intensity on the
above factors (velocity-light intensity,
degrees from normal-intensity).(21)

·The percentage of larvae showing a (a)positive,
(b)negative and (c)neutral response.(22) at the different
wavelengths and frequencies.

This last one will be in the form of a bar
chart of percentages; the others will be scatter graphs.

Justifications

These are the reasons that I made the choices
that I did. The numbers correspond to those in subscript in the plan.

1)The box is designed not to let any light from outside to
flood in. This could cause the maggots to respond to the external light, rather
than the intended source.

2)The scoop is to prevent injury to the maggot. If it is
damaged, its responses may be lessened. Tweezers and other metal implements,
fingers and spoons may easily cause injury. The scoop is designed not to.

3)Matt black is the most absorbent (least reflective)
surface. I will line the walls of the box with it to increase the light
intensity gradient. This also makes the light more one-directional. It must be
flat-bottomed to prevent the maggots from moving in a particular direction
purely because of a physical gradient.

4)Neon strip lamps emit much less heat than filament lamps,
as they are more efficient. This reduces the possibility of maggots responding
to I.R. stimulus.

5)The light meter is to ensure that when I double the
intensity, the spot that the maggot starts on is actually receiving twice the
intensity of light, otherwise the comparison of response to an increase is only
quantitative.

6)Stop clocks are an accurate and easy-to-read way of timing
the intervals between readings. They are simple to start and are free-standing.

7)The larvae need to be kept in the fridge during the two
days over which I will do the experiments. This is because the lower
temperature lowers the effectiveness of enzymes and therefore slows their metabolism
down, so that they do not start pupating and hatching during the experiment.

8)Maggots’ response to extraneous light could affect the
results. A way of minimising this is to work in a dimly lit room.

9)Only taking enough maggots out as will be used in the
experiments further slows their development into pupae.

10)If they are used cold (just after they have come out of
the fridge)

a)They
will be warming up , so there will be a difference between the results of each
individual maggot

b)They
will not be as active, so their responses may be too slow to be recorded or the
size of them may make them appear random.

11)Setting the variable resistor before the experiment starts
prevents having to set the value in the dark, or switching it on to find it is
on the wrong setting. Making sure that there is the same amount of light
falling on the larvae with all the filters took into account the varying
absorbency of the neon light (see point 14).

12)I have recorded the co-ordinates at different times to
enable me to

a)plot
graphs of the effect of wavelength and intensity on the rate of movement of
larvae and;

b)look
at the results afterwards and explain any anomalies by the overall behaviour of
the maggots, from beginning to end.

13)The more re-runs the better, but more than five would take
too long, and five is a good number because there is always a majority in the
number going in one direction.

14)Checking that the light intensities for all three filters
are the same is essential, as otherwise one cannot be sure whether they are
responding to the change in intensity or wavelength.

15)I thought this would be a suitable range, as too low an
intensity would get little response, too high an intensity may not be possible
with the same lamp, and may affect the behaviour of the maggots, e.g. by blinding
or dehydration them.

16)Having these measurements at different times gives a
better picture of the general pattern of movement of the larva. Discussed more
below.

17)The chi-squared test is a reliable test for statistical
viability, and can be done on a spreadsheet. The results will be mathematically
split up into these groups for easier graphical representation of the results.
This will not be done during the experiment, as only having a record of the
type of response of the larva gives little idea of where they have travelled
with time, and therefore what the real response was. For example, if they took
a long time to change direction but did eventually, this would not be shown if
only their final destinations were recorded.

18)Standard deviation can easily be applied to any size of
column or block of data, and the values can be compared easily.

19)This is to check my prediction that their response would
be more rapid to more damaging wavelengths.

20)This is to check whether it made a bearing on their
direction of movement: different direction is a different kind of response.
Non-detectable wavelengths might result in random direction, as predicted by
the null hypothesis.

21)This is to check my prediction that blowfly larvae would
"move away faster, the greater the intensity of the light" and
"move away at a smaller angle to the direction of the light, the greater
the intensity of the light".

22)This is again to check my prediction that they would
"respond more strongly to more damaging (higher frequency and therefore
energy) wavelengths, i.e. those closer to the Ultra-Violet end of the spectrum
of visible light". If this is true there will be less random or neutral
movement at the higher energy end of the spectrum.

Limitations and Precautions

These are the factors that could render my
investigation invalid, and the steps that I will take to minimise them

Heat pollution

Description: If there is a significant amount of I.R. radiation
emitted from the source of light or any other stationary source, it is possible
that the larvae respond to this rather than the visible light stimulus. This
could have a large effect.

Steps taken to minimise it: Measure the temperature at the starting
point for each of the different intensities to check whether it varies. Check every
ten centimetres across the floor of the box to check if there is any
difference. If so, record it next to the experiment starting at that point.

Variations in temperature

Description:
The temperature in the lab varies from day to day, and throughout the day. This
can not be avoided, but can be accounted for by taking the temperature every
time.

Steps
taken to minimise it: Take the temperature at the beginning of each
experiment, and record it in the space in the table provided. This does not
avoid the fact that, even after warming up for ten minutes they may still be at
a different temperature to the box. This would, however, not really matter as
all the maggots are in the box fr the same amount of time.

Light flooding

Description:
If stray light of appreciable intensity falls on the larvae, they may respond
to this rather than the one-directional strip lamp. This could cause the
results to be invalid, as they would not be affected by the lamp whose intensiy
is being measured.

Steps
taken to minimise it: I will avoid this problem by working in the dark and
with an enclosed box.

Pupating

Description: if the maggots get old enough, they may start to
develop into the pre-pupal stage and their behaviour may begin to change.

Steps
taken to minimise it: Keep the maggots in the fridge and only out for as
long as necessary. Buy fresh maggots near the time of execution of the
experiment.

Paper does not simulate meat very well

Description:
Meat is three-dimensional and maggots are three-dimensional. They have
three dimensional receptors and move in a three-dimensional way. The movement
that is being measured in this experiment is purely two-dimensional.

Steps
taken to minimise it: There is not much that can be done on this note,
apart from the darkness and one-directional light ensured by the black box.
Their movement on cardboard (in air) may be more difficult than that in a
firmer medium.

Small number of maggots causing fluctuations

Description:
The more samples are taken, the less the extent to which random results affect
the averages.

Steps
taken to minimise it: The effect of this can be kept to a minimal by
keeping the number of maggots the same. There is not enough time within the
scope of this experiment to repeat the experiment more than five times. Using a
different larva each time increases the chances of having random results but
decreases their affect as the other larva in the same experiment show it to be
anomalous.

Maggot injury

Description:
If a larva is handled carelessly or are used for many consecutive experiments,
it may become damaged and no longer act in a manner typical of other larvae.

Steps
taken to minimise it: using the scoop, using a different larva for each
experiment and not dropping them should minimise this.

Safety Risks

·As we will be working in the darkness, to prevent
accidents bags must be kept under desks, and the floor spaces must be clear.

·Wash hands after handling blowfly

·Do not allow the colour filters/box to become
overheated, as there is a fire hazard with the cardboard box.

·Good laboratory practise- making sure implements are
put in the right places, wires of apparatus are insulated and away from water,
and so on.

·Prevent maggot loss by having tightly enclosed box

Modifications

As it was difficult to ensure that the larvae faced
in the same direction until I started the stopwatch, I recorded the
direction that they were facing on an extra column on the table.

The room could not be as dark as I had hoped.

I moved to a smaller room with less people to cut
down the variations in light levels from many peoples' experiments.

I found that the variable resistor had no effect on
the intensity of the neon strip lamp, due to its internal electronics. I
therefore moved the starting point for the maggots to vary the intensity
and recorded the relative intensity on the light meter, as well as their
starting point, in an extra column on the table.

I had to use a camera as a light meter, as the one
that I was planning to use (a ‘Logit box’) was insufficiently sensitive.

It was therefore not able to measure 2 times the
light intensity, three times the light intensity etc , but increased the
light level according to the sensitivity of the camera’s meter.

I decided to make the positive
responses red on the table, the neutral
responses blue for easier reading. Note: a negative response is taken to be away from the light and
therefore a negative response to light (negative phototaxis/photokinesis).
Henceforth when I refer to "negative" and "positive"
this is what I mean.

Results:

The results are shown on the next
few pages. First comes the main results page.

[To download the results in Microsoft Excel 95 format, click here, email me
to request for other formats. This digital version does not include trend
lines, which must be included on the graphs to show correct interpretation]

Errors, limitations and Anomalies Affecting Results

Errors that could have been introduced by my
method are:

·Time inaccuracies
are unlikely due to the use of a stopwatch. There could have been some error
introduced by the slightly varying time lag between reading the time on the
clock and noting the co-ordinates of the larva.

·Parallax error
could have been a factor, as taking readings from a small hole in one end of
the box meant that the angle at which I viewed the larva changed as it moved,
so the parallax error changed during each experiment, meaning that there may be
slight inaccuracies in the readings.

·Percentage
inaccuracies were fairly high, as the measurements were only to the nearest
centimetre and the distances between one and twenty centimetres.

All of these factors were
counteracted by using five maggots and taking the averages of the measurements
at 10, 20 and 30 seconds.

The limitations and errors that could have
affected my results are:

Pupating

Although only about five to ten percent of the maggots used
were actually seen to be changing colour, it is very possible that a larger percentage
were tending towards the pre-pupal stage, and their responses were beginning to
change. This would explain the fact that, in all samples, there are some
showing positive phototaxis, whereas the majority showed a negative response.

Heat pollution

The table of my results from
measuring the variation of temperature within the box is shown below.

Control: temperatures table

Temperature

Distance from light/cm

17

5

16

10

17

15

17

20

17

25

16

30

16

35

17

40

This table shows little
variation. Any variation could have been caused by random fluctuations in the
circuitry of the Logit box or a borderline temperature of 16.5 and random fluctuations
in air temperature within the box. There is no trend with the distance away
from the light (although I left it on for 10 minutes before taking these
measurements) and therefore it can be concluded that there was no influence on
the maggots of this type.

Variations in temperature

The temperatures at the time of experimentation
are recorded on the table of results. There is little fluctuation.

Light flooding

As this appeared to be a potential problem, I
moved to a smaller room with less people, as stated in the modifications.

Paper does not simulate meat very well

This is definitely a point for consideration.
Although this experiment proves the responses of maggots to light in this
experiment, the differences between this experiment and the natural habitat of
the Blowfly must be considered before it can be concluded that this behaviour
is typical of all blowfly everywhere.

Small number of maggots causing fluctuations

Although the more samples there are the more
sure one can be, it appears that five was a big enough sample to adequately
investigate their responses.

Maggot injury

As a special scoop as described in the plan was used for
this experiment, it is unlikely that this was a factor, although this could be
an explanation for a few anomalies, as there could have been some injuries from
their being in a large container with all the other larvae and could have been
injured at this stage.

The main anomalies for consideration are:

·Larva number 4, at
wavelength 460nm: This showed a neutral response, moving at 77 degrees to the
normal.

·Larva number 4, at
wavelength 570nm: This showed a neutral response, moving at 66 degrees to the
normal.

·Larva numbers 4&5,
at relative intensity 1: These showed a neutral response, moving at 84 and 61
degrees to the normal.

·Larva numbers 2&3,
at relative intensity 0.67: These showed a neutral response, moving at 68 and
63 degrees to the normal.

·I would have said Larva number
5, at wavelength 570nm, Larva number 1,
at wavelength 650nm, Larva number 1, at relative
intensity 3, Larva numbers 1&4, at relative
intensity 2, Larva numbers 4&5, at relative
intensity 1.43, Larva number 3, at relative
intensity 1, and Larva numbers 1&5, at
relative intensity 0.67, as they all showed a positive response, but the
large number indicates a rule rather than an exception.

Conclusion

I found that
the blowfly larvae demonstrated an innate negative phototactic response.

I found that
an increase in the intensity of light did cause an increase in the phototactic
response of blowfly larvae, but an increase in frequency of radiation did not
produce such definite results.

·Graph One:
The graph of average rate of movement against wavelength showed that:

a)The
larvae responded with most consistency to green light (570nm), although there
was an anomaly with maggot five. This maggot moves very quickly sideways, but
did not move much along the y-axis. It was possibly injured or otherwise just
an anomaly.

b)The
other wavelengths produced much more spread-out results- possibly a sign that
the larvae are not sensitive to these frequencies.

·Graph Two:
The graph of angle to normal against wavelength, however, showed more response
to the other frequencies, but a extreme response for green. For example, the
angles for blue ranged between –77.47 and 45
degrees, but for green they ranged between –41.99
(maggot five) and 66.04 degrees (maggot four), nearly staying completely within
the ‘positive’ zone. This could show that the larvae are particularly sensitive
to green light. The idea is backed up by my result for standard deviation- the
deviation of movement for green light, at 0.31, is lower than that for the
other frequencies.

The reason for this could be that, although higher frequency
wavelengths are more damaging, they are not normally in the incident light
falling on the larvae, as blue light is scattered more easily. Red light is
barely detectable if the larvae are under vegetation, so green light is the
important one. My results, however, do not differ so much between the three
wavelengths that I tried, so the larvae may not be (so) sensitive to different
wavelengths.

·Graph Three:
The graph of the rate of movement against the relative light intensity showed
that the rate of movement is related by direct proportionality to the intensity
of the light i.e., that the larvae demonstrated a phototactic response. This
graphical observation is backed up by the standard deviation values, which are
smallest for the lowest intensities, as the distances moved are less, so the
extremes of movement are closer together. But the larvae were not always going
away from the light: for example maggot one for the relative intensity of 3.
This also demonstrates another point.Whilst the larvae that moved towards the light at some intensities (e.g.
maggot 3 at relative intensity 1) appear to be wondering about slightly
randomly, maggot one at intensity 3 fits into a steady negative pattern: they
show both negative
and a positive phototactic response (see below).

·Graph Four
The graph of the angle to the normal against relative light intensity showed
this again: the more intense the light, the more focused the direction of the
movement of larvae (the less got lost).

This can be explained by looking at the
lifecycle of the blowfly in more detail. When the larvae need to develop into
pupae (which are camouflaged brown and need to be on the surface so that the
hatched flies (as shown in Figure 5)
can fly away), directional movement will again be of great advantage, this time
towards the light source. Photokinesis will again be of little value here, as
in a large carcass it would take a long time to find the light, as random
movement could result in going further down. So, although the larvae
need to get in to the food to feed and protect itself whilst doing so, when it
is ready to pupate, it must get back to the surface of its food to be
able to hatch out of its pupal stage as a fly. The changeover in negative and
positive phototaxis would enable this most efficiently as the larvae’s needs
changed from needing to get away from the lighter surface, to needing to
be on it. The phototaxis rather than photokinesis is the most efficient
and inconspicuous way of doing this, as the goal is always either straight
towards, or straight away from, the light.

·Graphs Eight
to Twelve: These show that for the highest intensities, the largest
proportion always demonstrated negative phototaxis. For intensity 1, the
negative was the same as the neutral response at 40%. For intensity 0.67, the
positive and neutral responses were larger than the negative, both including
40% of the larvae. This seems to suggest a threshold intensity: if the light is
not of a minimum intensity, the larvae may wander round, but not detect the
light sufficiently to be able to determine which direction to go in. They
therefore walk in a straight line (they start facing the side) and record as a
neutral response, or change direction for a slightly negative or positive
response. The reason that there appears to be a larger number of these
"wanderers" at low intensities could be either:

·that there are more larvae at this intensity that can't
detect the light, therefore more demonstrate a random response

·that the larvae at intensity 0.67 moved complete
randomly, and could not detect the light at all

The second suggestion is supported by the fact that the
average rates of movement for the five larvae at intensity 0.67 were 0.1, 0.0,
0.0, 0.0, and 0.1. In other words, it seems very obvious that either the larvae
could not actually detect the light at relative intensity 0.67, or their light
sensors at this level were not sensitive enough to detect the difference
between the level of light on their left and right sides.

It could be an advantage that their sensors are only
triggered by certain levels of light, as, once they are far enough inside their
piece of meat (or whatever), they need to stop and start feeding, so that they
can start storing energy as soon as possible before they go into pupation. If
they started running around at the first hint of light, they might spend all
morning trying to get to the middle of the food, as when the sun rose all the
larvae would fight for pole position! Evolution has therefore favoured a
threshold frequency at an appropriate level, and the nervous response is only triggered
when the stimulus is strong enough.

·Graphs Five to
Seven: These all show negative phototaxis as the primary response. The
nature of these type of graph combined with only having five repeats tends to
exaggerate small results, for example having one little insignificant larva
going a different direction means that it shows a block a fifth of the total
height of the graph- not easily missed.

Conclusion Evaluation

Despite the anomalies discussed on page 10, I
think that my conclusion is a valid one. The evidence for the existence of both
negative and positive phototaxis, and the predominance of negative phototaxis
is strong.

The chi-squared test, as shown below,
demonstrates that the result of the variation between the results and those
predicted by the null hypothesis was much greater than 3.84, so the null hypothesis can be rejected. Although the number was not
so different for positive, there was a much smaller number of neutral responses
than predicted, and a much larger number of negative responses.

Control:
Chi-squared test table

Type of
movement

Observed
(O)

Expected
(E)

O - E

((O - E )^2)/E

Positive

10

13

11

8.53

Negative

24

13

-3

0.83

Neutral

6

13

-7

4.03

Total

13

The
evidence for the existence of a link between wavelength and sensitivity of the maggots
is, albeit weaker, still evident in all forms of analysis (angle to normal,
average rate of movement, and standard deviation), so I still think that there
is a link. This could, however, be investigated further, perhaps with a prism
to be able to vary the spectrum accurately. There were no known sources of
chemical pollution in the box. The only possibility is the Sellotape®. This
could have been detected by the chemoreceptors on the blowfly larvae’s legs (as
shown in Figure 6). This, however,
would have made any increase in light intensity have no effect on the larvae's
movement, and is therefore unlikely.

As the
responses are innate (instinctive nervous responses rather than
conditioned/learned), they would not vary between Blowfly Larvae. I think that,
despite the differences between the natural and tested mediums, the conclusion
is valid, as a negative phototaxis would only be more strongly demonstrated in
an environment where it has evolved to move. Therefore it is safe to conclude
that this study is representative of all Blowfly Larvae.